Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder & Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

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754 USER COMMENTS

Bennie, bond strength can be tested by using ASTM C1583 / C1583M - 13 Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-off Method). When the test is performed on the surface of a repair or an overlay material, it determines the bond strength to the substrate or the tensile strength of either the overlay or substrate, whichever is weaker. You can look up the method online, but you will need some a tensile pull tester to actually perform the test. Some also use a sound test to determine the bond using a hammer or chain. If you tap the floor with a hammer you can hear the hollow spots if there are any. Dragging a chain across the floor can also detect poorly bonded concrete surfaces. A distinctly hollow, drum-like sound is heard when delaminations are encountered.
- Lee-Technical Service
Wednesday, April 5, 2017 at 9:19 AM

Is there a test to verify the bond strength between layers?
- Bennie
Tuesday, April 4, 2017 at 2:44 PM

Tom, yes you can put another layer on the existing layer. You will need to wait 28 days from the time the first layer was installed. Clean the surface and then apply the Bonder Fortifier and your stucco after the bonder has dried. You will then need to wait 7 days to paint with a latex based paint or 28 days for anything other than latex.
- Lee-Technical Service
Monday, April 3, 2017 at 10:05 AM

I have recently had my retaining wall top cleaned bonding agent applied and re stuccoed. since I still have bonding agent left, can I apply another coat then paint the top of the wall with a good concreate paint??
- tom
Sunday, April 2, 2017 at 12:02 PM

Kelly, you will need to grind the surface and make sure there are no paints, sealers, oil, dust, and or debris. Once this is done then pressure wash really well. Then while the surface is damp use the Top'n Bond Concrete Patcher. This material can be placed from 1/2" down to a featheredge.
- Lee-Technical Service
Tuesday, March 28, 2017 at 12:29 PM

I have existing concrete pad that is low in the one corner. I would like to extend the pad. The corner that is low, can I use Flo-Coat to bring the corner up and they pour concrete next to the existing Pad include the corner with the Flo-coat.
- Cliff
Sunday, March 26, 2017 at 12:34 PM

There is a low spot about 1/2" in our garage where our garage door shuts leaving a small gap on the cement floor. How would you recommend getting a good bond to raise that small portion with cement?
- Kelly
Wednesday, February 22, 2017 at 4:46 PM

Richard, you will need to follow the TCNA guidelines for installing radiant heat flooring. TCNA requires 3/4" over the top of the hydronic tubing when it is being installed over concrete. However, you will need to confirm this with your installation using the TCNA manual. We do not have a product that can be used for this application at this time.
- Lee-Technical Service
Wednesday, February 22, 2017 at 12:54 PM

James, you can simply use any of the concrete mixes to fill the hole. Just clean out the hole, mix the material and pour. However, my biggest concern would be code. You need to get a recommendation from your local code department so that your project stays within code compliance. If you were to just fill the hole and replace the stud, this might not be compliant and they would make you rip it out and start over. So just check with the local code department so that you don't waste any time and money.
- Lee-Technical Service
Wednesday, February 22, 2017 at 12:43 PM

I am looking to add a 1" layer of rigid foam over an existing concrete floor to allow for radiant floor tubing. What is the minimum thickness pour needed to cover my tubing and allow for ceramic tile floor?
- Richard
Tuesday, February 21, 2017 at 2:07 PM

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